This invention relates to an optical bistable device (OBC) for inherent potential applications such as optical switching, optical memory, bistable logic, differential amplification, optical transistor, discrimination, clipping, limiting, pulse shaping, and for performing a number of optical digital data processing functions. More specifically, the present invention relates to an optical bistable device in which an active laser medium is placed between two mirrors within an optical resonator whereby bistable switching is provided by operating the active resonator across the transition borderline between the stable-unstable resonator regions.
In conventional optical bistable devices (OBC), nonlinear Fabry-Perot resonators containing saturable absorbers or nonlinear refractive-index material are utilized. A schematic diagram of such a device is shown in FIG. 1(a) wherein a laser beam is used for power input. Typical characteristics of such an optical bistable system are illustrated in FIG. 1(b).
This type of optical bistable device is referred to as "intrinsic system". U.S. Pat. No. 3,610,731, issued on Oct. 5, 1971 to H. Seidel and U.S. Pat. No. 3,813,605, issued on May 28, 1974 to A. Szoeke, both disclose an intrinsic system containing saturable absorbers. Nonlinear Fabry-Perot resonators containing nonlinear refractive index materials were demonstrated and described by Gibbs et al in 36 Phys. Rev. Lett. 1135 (1976). The phenomenon appearing in one of such intrinsic systems is described in "Laser Focus", April 1982, page 79, which is incorporated herein by reference.
Another type of optical bistable device involves a so-called hybrid system. In contrast to the intrinsic systems mentioned above, the microscopic nonlinearity in hybrids is synthesized using electro-optic feedback. The hybrid optical bistable containing a medium with a nonlinear refractive index and an electrical feedback loop was initially suggested and experimentally illustrated by Smith and Turner in 30 Appl. Phys. Lett. 280 (1977). An example of an electro-optic hybrid analogue of a dispersive optical bistable device is shown in FIG. 2. The operating principle of such systems is described in "Laser Focus", April 1982, page 81, which is incorporated herein by reference.
As mentioned above, various types of optical bistable devices which have so far been demonstrated in either hybrid or intrinsic systems require nonlinear refraction as the microscopic nonlinearity. For intrinsic optical bistable systems, an external laser is generally required for operation because of the need for high intensity and/or interference, while hybrid systems which could be driven by a broad band source require electro-optic feedback circuits.
More specifically, in the nonlinear Fabry-Perot resonators containing saturable absorbers, which can be referred to as absorptive OB, comparatively large changes in absorption are required to give rise to absorptive bistability. Generally, it is difficult to produce absorptive OB because the absorber must saturate to a low level of residual absorption as described in "Laser Focus", April 1982, on page 81.
The Fabry-Perot resonator containing nonlinear refractive index material, which can be referred to as dispersive OB, requires nonlinear media showing a large intrinsic nonlinear index of refraction which is difficult to find. In addition, those systems requiring multiple-beam interference need relatively coherent light and hence will generally require external lasers for their operation.
For the hybrid optical bistable system, since the microscopic nonlinearity required in the cavity is synthesized by using electro-optic feedback, the switching speed of the device is generally limited. In addition, an external laser beam is required in most of the hybrid optical bistable systems.
Now, in the study of input vs. output power characteristics of a flashlamp-pumped Nd:GGG rod geometry laser, the present inventor has observed that the output laser power increases with increasing flashlamp input power. As the flashlamp input power exceeds a certain power level, the laser output power is reduced to an insignificant level and laser action ceases if the input power is further increased. At that stage, the flashlamp input power is reduced and no laser action is observed until the input power decreases to a second power level which is smaller than the certain power level. As the input power is further reduced below the second power level, laser action resumes. A hysteresis effect is reproducible.
This OB phenomenon has not been noticed so far in prior art laser resonators, which may be because of the fact that laser operation in the region of stable-unstable configuration transition has never been carefully studied.
The present discovery led to the development of a more general concept for active optical bistable laser devices, as described herein.